1. Field of the Invention
The present invention generally relates to devices for the control of flow of liquids, gases or mixtures thereof in a fluid containing passage and, more particularly, to throttle valves presenting a controllably variable area to a flow of pressurized fluid.
2. Description of the Prior Art
Many arrangements are known which require control of the flow of a fluid, ranging from simple household water distribution systems to complex power plants and motive power systems and pneumatic or hydraulically operated controls for heavy machinery. Control of flow of either liquids or gases or mixtures thereof, such as in heat exchange systems, may be required. To achieve this control many types of valve of highly diverse geometries have been developed.
Many such applications of valves involve highly pressurized systems, often causing high velocities and large pressure drops across the valve or parts thereof, particularly when constricted. Such large pressure drops may cause vapors to form within a flowing liquid or cause entrained gases to separate from the fluid. This formation of gaseous phase within a liquid due to pressure change is referred to as cavitation.
Such cavitation places large and unpredictable forces on valve parts and may cause mechanical erosion of valve parts or the valve interior, requiring periodic maintenance. Further, cavitation can be a major source of objectionable noise due to fluid flow or pressure release. Depending on the pressure drop and the structure of the valve, cavitation noise can reach extremely high levels, often requiring acoustic treatment for control.
Some designs of so-called non-cavitating valves are known in the art. These designs generally involve numerous long, narrow passages for fluid flow. The control of fluid flow through such a valve is achieved by control of the number of such passages through which flow is permitted, often with a sliding gate or movable plug structure. However, cavitation is often not entirely prevented since a sliding gate will allow some of the fluid flow passages to be only partially opened and cavitation will usually occur therein.
Further, with such a type of valve geometry, the length and narrowness of the passages and the viscosity of the fluid limits the volume of flow relative to the overall dimensions of the valve and causes a significant "wide-open" pressure drop even at moderate flow velocities. By the same token, the fluid flow cross-sectional area is also relatively limited compared to the overall dimensions of the valve. Further, the narrowness of the passages makes them subject to partial or total blockage due to capture of contaminants and particulates which may be present in the fluid, thus also contributing to the need for periodic maintenance and unpredictable failure. Additionally, such valves must be highly precise and internal parts are typically provided as matched sets.
The use of springs of various forms in valve structures is known for a variety of purposes. For example, in Japanese Patent 61-124790, a helical spring is used to provide a load force against a valve plug to reduce vibration of parts. Helical springs are also known as a combined fluid control element and pressure sensor for pressure relief as in U.S. Pat. No. 2,369,005 and Soviet Union Patent SU-1302-069-A or for flow regulation as in U.S. Pat. No. 4,858,644. However, for full shut-off, treatment of the surface of the helical spring such as by coating with a resilient material is necessary. This treatment is subject to damage and wear and also requires periodic servicing by retreatment at significant expense and constitutes a failure mode of the valve if full shut-off is required. Additionally, the flow regime between coils of a helical spring is essentially that of a high aspect ratio duct which has little effect on prevention of cavitation and limits applications to low flow rate requirements and/or the maintaining of a substantial pressure drop across the valve, such as in the pressure relief applications mentioned above.